Perl Programming/Keywords/BEGIN

The BEGIN keyword
BEGIN is one of five code blocks that are executed at the beginning and end of a program by Perl. It is create the block that is executed ASAP after the Perl interpreter starts. More than one BEGIN block may exist within a file or an eval'd string, and they are executed in order of definition.

Although it can be prefixed with sub, this is not considered a good programming practice, as these blocks are not a subroutine.